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Ecological Importance of Snowbrush Ceanothus Velutinus in the Oregon Cascades

Zavitkovski, J., Newton, M.
Ecology 1968 v.49 no.6 pp. 1134-1145
Ceanothus velutinus, Pseudotsuga menziesii, bioassays, biomass, greenhouses, nitrogen, nitrogen content, nitrogen fixation, nodulation, organic matter, seedlings, seeds, shrubs, soil, vegetation, Oregon
Twelve stands of snowbrush were selected for a biomass study to estimate nitrogen fixation under field conditions west of the summit of the Oregon Cascades. Total nitrogen in the upper 2 ft of soil was higher under snowbrush than in the open, but the difference may have been caused by loss of nitrogen from open areas rather than by nitrogen fixation. Total nitrogen in the upper 15 cm of soil under snowbrush did not differ from that found under nonfixing shrub species. Various shrub species may increase the total soil nitrogen under their canopies, however, if only by accumulation from sites that lack vegetation. More nitrogen may be tied up in the biomass of mature snowbrush stands than in stands of other shrubs. The difference could be explained by nitrogen fixation, which may range from zero to about 20 kg/ha per year under conditions of this study. Nodulated snowbrush seedlings produced 2.5 times the dry weight that non—nodulated seedlings produced in a nitrogen deficient soil. Sixty—one per cent of the nitrogen in nodulated greenhouse seedlings was fixed. Such fixation rates may be reached on infertile soils in the field, but they seem unlikely on soils of medium or better fertility. This point seems to be substantiated by delay in nodulation of snowbrush in soils with increased levels of organic matter. Bioassay tests using Douglas fir seeds and hemlock seedlings showed that snowbrush did not add to the soil any significant amount of nitrogen by nitrogen fixation. The species contributes to the formation of a new organic layer, however, through large amounts of nitrogen—rich litter.